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COMP 110-001
Computer Basics
Yi Hong
May 13, 2015
Today
 Hardware and memory
 Programs and compiling
 Your first program
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Before Programming
 Need to know basics of a computer
 Understand what your program is doing
 Talk intelligently about computers
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Computer System
 Hardware: Physical
components for computation
• CPU, Memory, Keyboard ….
 Software: Programs that give
instructions to the computer
• Windows, Office, Games,
Eclipse …
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Hardware
 Main components of a computer
• CPU (Central Processing Unit): Performs the
instructions in a program
• Memory: Holds programs and data
• Input devices: Provide data to a computer
• Output devices: Display data carried out by a
computer
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CPU – the “Brain”
 Central processing unit
• Clock speed: GHz, how many clock
cycles a CPU can perform per second
(1GHz = 1 billion CPU cycles per second)
• Dual core: Multiple processing
units per CPU
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Memory – the Brain
 Holds data for the computer
 Main memory
• Holds the current program and much of the data that
the program is manipulating
• Volatile, disappears when shutting down the
computer
 Auxiliary memory (secondary memory)
• Hard disk drives, CDs, flash drives …
• Exists even when the computer’s power is off
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RAM (Random Access Memory)
 The main memory
 4 gigabytes of RAM
• A bit: the basic unit of information in
computing (binary digit, 0 or 1)
• A byte: A quantity of memory, 8 bits
2^0 + 2^2 + 2^4 + 2^5 = 53
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Measuring Memory
 Both main memory and auxiliary memory
are measured in bytes
• 1 byte = 8 bits
• 1 Kilobyte (KB) = 1024 bytes
• 1 Megabyte (MB) = 1024 KB = 1024*1024 bytes
• 1 Gigabyte (GB) = 1024 MB
• Terabyte (TB), Petabyte (PB) …
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Software
 Program: A set of computer instructions
Program
Data (input for
the program)
Computer
Output
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Programming Languages
 Different levels
Java / C++ Program
High-Level Languages
(Human readable)
Compiler
Assembly languages
Machine Language
Low-Level Languages
(Computer readable)
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Translation
 A high-level language ? a low-level
language
• Compiler: translate once, run forever
• Interpreter: translation alternates with
execution, directly executes instructions
 Java: combines a compiler and an
interpreter
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Java Bytecode
Java Code (.java)
A compiler
translates Java
code into bytecode
JAVAC
compiler
Java Bytecode (.class)
JVM
JVM
JVM
Windows
Linux
Mac
The Java Virtual
Machine (JVM) is
an interpreter that
translates and
executes bytecode
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Why Using Java Bytecode?
 The bytecode is not the machine language
for any particular computers
 Can be easily translated into the machine
language of a given computer
 Portability
• Java bytecode runs on any computer has a
JVM
• No need to recompile the Java code
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Objects, Methods, and Classes
 Object: a combination of attributes (data)
and methods (actions)
• Yi’s Car (has wheels, can start, stop, …)
 Class: defines a type or kind of object
• Car (blueprint for defining the objects, e.g.,
Yi’s car, Bob’s car …)
 Methods: actions performed by objects
• start(), stop(), forward(), back() …
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Invoking a Method
 A Java program uses objects to perform
actions that are defined by methods
Yi’s car.forward ();
Semicolon at the end of
each statement
System.out.println(“Welcome to COMP 110”);
Object to
perform actions
Method of the
object System.out
Argument
• Print the string in quotes to screen
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First Java Program
 A simple task
• Print a message: “Welcome to COMP 110”
Each class is in *.java
A class contains methods
Every application
has a main method
The body of the method
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Begin the Program
 Begin a program named “FirstProgram”
 Program names should make sense
 Capitalize the first letter of each word in
the program name
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Run the First Program
 Compile: javac FirstProgram.java
• Bytecode is in the file, FirstProgram.class
 Execute: java FirstProgram
 Or use IDE (integrated develoment
environment)
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Second Java Program
 Ask the user to input his/her name, and
print a welcome message
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What’s New (1): Java Package
import java.util.Scanner
 Gets the Scanner class from the package
java.util
 Package = Library of classes
 Different libraries provide different
functionalities
• Math library: math equations
• Network library: send / receive packages
• java.util : allows you to read data from keyboard
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What’s New (2): Create Objects
Scanner keyboard = new Scanner(System.in)
• Create an object (i.e., keyboard) of the
Scanner class
• Then the object performs actions:
String name = keyboard.next();
• Read a string from the keyboard
Keyboard.close();
• Close the keyboard, stop getting data from a user
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First & Second Java Programs
 Import a package / class
 Define a class
 A main method
 Create an objects
 Invoke methods
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Next Class
 Object-Oriented Programming (OOP)
 Primitive data types and variables
 Reading and coding assignments
• Chapter 1.3, 2.1
• Try the two sample programs in class
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